﻿    <p>
      <em>IfcGrid</em> ia a planar design grid defined in 3D space
      used as an aid in locating structural and design elements.
      The position of the grid (<em>ObjectPlacement</em>) is defined
      by a 3D coordinate system (and thereby the design grid can be
      used in plan, section or in any position relative to the
      world coordinate system). The position can be relative to the
      object placement of other products or grids. The XY plane of
      the 3D coordinate system is used to place the grid axes,
      which are 2D curves (for example, line, circle, arc,
      polyline).
    </p>
    <p>
      The inherited attributes <em>Name</em> and <em>Description</em>
      can be used to define a descriptive name of the grid and to
      indicate the grid's purpose. A grid is defined by (normally)
      two, or (in case of a triangular grid) three lists of grid
      axes. The following figures shows some examples.
    </p>
    <p>
      A grid may support a rectangular layout as shown in Figure 5, a radial
      layout as shown in Figure 6, or a triangular layout as shown in Figure 7.
    </p>
    <blockquote class="note">
      NOTE&nbsp; The <em>PredefinedType</em> denotes the type of
      grid that is represented by <em>IfcGrid</em>. The
      instantiation of <em>IfcGridAxis</em>'s has to agree to the
      <em>PredefinedType</em>, if provided.
    </blockquote>
    <blockquote class="note">
      NOTE&nbsp; The grid axes, defined within the design grid, are
      those elements to which project objects will be placed
      relatively using the <em>IfcGridPlacement</em>.
    </blockquote>
    <table cellpadding="2" cellspacing="2">
      <tbody>
        <tr>
          <td width="320">
            <img src="../../../figures/ifcdesigngrid-type1.gif" alt="1"
            border="0" height="211" width="306">
          </td>
          <td width="320">
            <img src="../../../figures/ifcdesigngrid-type2.gif" alt="2"
            border="0" height="211" width="306">
          </td>
          <td width="320">
            <img src="../../../figures/ifcdesigngrid-type3.gif" alt="3"
            border="0" height="211" width="306">
          </td>
        </tr>
        <tr>
          <td width="320">
            <p class="figure">Figure 5 &mdash; Grid rectangular layout</p>
          </td>
          <td width="320">
            <p class="figure">Figure 6 &mdash; Grid radial layout</p>
          </td>
          <td width="320">
            <p class="figure">Figure 7 &mdash; Grid triangular layout</p>
          </td>
        </tr>
      </tbody>
    </table>
    <blockquote class="history">
      HISTORY&nbsp; New entity in IFC1.0.
    </blockquote>
    <blockquote class="change-ifc2x4">
      IFC4 CHANGE&nbsp; The attribute <em>PredefinedType</em> has
      been added at the end of the attribute list.
    </blockquote>
    <p>
      &nbsp;
    </p>
    <p class="spec-head">Informal Propositions:</p>
    <table summary="IP">
      <tr>
        <td width="640">
          <ol>
            <li>Grid axes, which are referenced in different lists
            of axes (UAxes, VAxes, WAxes) shall not be parallel.
            </li>
            <li>Grid axes should be defined such as there are no
            two grid axes which intersect twice (see Figure 189).
            </li>
          </ol>
          <blockquote class="note">
            NOTE&nbsp; Left side: ambiguous intersections A1 and
            A2, a grid containing such grid axes is not a valid
            design grid;&nbsp; Right side: the conflict can be
            resolved by splitting one grid axis in a way, such as
            no ambiguous intersections exist.
          </blockquote>
        </td>
        <td align="right" valign="top" width="320">
          <img src="../../../figures/ifcdesigngrid-ip2.gif" alt="IP2"
          border="0" height="97" width="306">
          <p class="figure">Figure 8 &mdash; Grid intersections</p>
        </td>
      </tr>
    </table>